Synthesizing the top level design with the "Create Schematic" Option selected causes the synthesizer to stop with the following error message:
runtime error R6018 - unexpected heap error
This can happen if you are using the older version (v3.1.0) of Viewgen shipped with PROseries. Both PROseries and VIEWsynthesis ship a viewgen.exe file, and if PROSER is first in the PATH, the older version of VIEWGEN is going to be used.
Old version/Bad one is: viewgen - V3.1.0 ; Workview 5.2 New version/Good one is : viewgen - v3.5 ; VIEWlogic 5.3.3
The solution is to either change your PATH and put VIEWSYN first in your PATH variable, or alternatively you may deselect the "Create Schematic" option in Viewsynthesis.